Steel mandrel plug and method of treating



United States Patent Office 3,496,031 Patented Feb. 17, 1970 3,496,031 STEEL MANDREL PLUG AND METHOD OF TREATING Paul J. Suhy, White Oak Borough, Pa., assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Oct. 17, 1966, Ser. No. 586,983 Int. Cl. C21d 9/00; C22c 39/54 U.S. Cl. 1486.35 7 Claims ABSTRACT OF THE DISCLOSURE An improved mandrel plug for the production of seamless pipe consisting of 1.1-to 1.4% carbon, 6.5 to 8.5% nickel, 18.0 to 20.0% chromium, 1.0 to 1.5% molybdenum, 0.5 to 1.0% copper, 0.5 to 0.8% aluminum, 0.7 to 1.3% silicon, 0.4 to 0.6% manganese, up to 0.05% phosphorus, and up to 0.05% sulfur. The mandrel is heated to a temperature above 2100 F. to form scale on the surface thereof, air cooled to below 900 F., reheated to above 1525 F. to austenitize the steel, and then air cooled for handling.

This invention relates to steel mandrel plugs for seamless pipe rolling. More particularly, the invention relates to a method of treating a steel mandrel plug to increase its usable life and to a steel plug produced thereby.

According to the invention, there is provided a method of treating a steel mandrel plug which provides a plug with improved properties and a comparatively long useful life. A steel mandrel plug in accordance with the invention consists essentially of 1.1 to 1.4% carbon, 6.5 to 8.5% nickel, 18.0 to 20.0% chrominum, 1.0 to 1.5% molybdenum, 0.5 to 1.0% copper, 0.5 to 0.8% aluminum, 0.7 to 1.3% silicon, 0.4 to 0.6% manganese, up to 0.05 phosphorus and up to 0.05% sulfur. The method comprises heating a steel plug of the aforementioned composition to a temperature above about 2100 F., preferably about 2100 to about 2400 F., to form scale on the surface thereof, air cooling to below about 900 F., reheating to above about 1525 F. to austenitize same, preferably about 1525 to about 1600 F., and then air cooling to a temperature for handling. The air cooling is advantageously performed in a quiescent atmosphere, i.e. Without fan cooling.

In a preferred embodiment, a steel composition is used for the mandrel plug which consists essentially of 1.3 to 1.4% carbon, 7.0 to 7.5% nickel, 19.0 to 19.5% chromium, 1.0 to 1.5% molybdenum, 0.65 to 0.8% copper, 0.5 to 0.7% aluminum, not more than about 1.0% silicon, not more than about 0.5% manganese and not more than about 0.05 each of phosphorus and sulfur.

A mandrel plug for seamless pipe rolling is a hot metal working tool used on the inside of pipes in seamless pipe rolling. During use, the tool is subjected to heavy shock and contact with hot metal under pressure. This requires material which has good toughness and wear resistance properties at the temperatures involved, usually in the range of 1800 to 2000 F. It is also necessary that the plug does not spall, break or crack when the Working surface is heated to such high temperatures while the interior portions of the tool remain at relatively lower tempreatures. Such tools should also be relatively insensitive to changes in properties after long excessive heating to the conditions to which it is exposed and used.

The mandrel plugs produced in accordance with the invention exhibit a high resistance to heat checking and radial cracking and very good toughness at hardnesses of 390 to 450 B.H.N. The superior properties of this mandrel plub help reduce loss due to breakage. A problem with mandrel plugs in the past has been a poor surface life in use in rolling hot tubes for tube rolling mills. This is particularly true of plugs used in rolling light wall pipes, i.e. pipes of thin wall thickness. The mandrel plug according to the present invention is superior to the best plugs heretofore used and gives an increased surface life and results in pipe productions of improved quality.

The present invention provides a particular steel alloy composition and heat treatment thereof which combines to provide a steel mandrel plug of increased usable life and which possesses the aforementioned desirable properties. Steel mandrel plugs produced in accordance with the invention have a greater resistance to wear and do not gouge as readily as other tools. The reduced tendency for scale pick up on the plugs outside diameter reduces pipe scoring and improves the quality of the inside diameter of seamless pipes produced through the use of the improved tool.

A mandrel plug in accordance with the invention is made from a nickel-chromium-molybdenum steel containing nickel, molybdenum and copper such that the steel is a stable austenitic alloy. In a preferred embodiment, steel of the aforementioned composition is cast in green sand or shell cast to size in sand or synthetic resin molds. The cast plugs are then ground or machined to a desired outside diameter of the working surface. The aforementioned composition is preferably made in an electric furnace, which is tapped at a high temperature, e.g. 2950 F, to control grain size and discourage segragation of the particles. Grain size control is further assisted by the additions of copper and aluminum.

As a specific example, the following was charged to an electric furnace: 2200 lbs. of worn mandrel plugs. 650 lbs. scrap stainless steel (18% chromium and 8% nickel),

and lbs. ferrochrome. After melting, the furnace temperature was raised for better casting, e.g. to approximately 3000 F. Silicon and manganese were added to insure deoxidation and, when the temperature dropped to 2950 F., the alloy was poured into a ladle and aluminum was added. The cast was made at a temperature between 2825 and 287-5 F. Smaller pipe plugs were poured at the higher end of the temperature range and larger ones were poured on the lower end of the temperature range. This procedure helped control fluidity in small areas. The stable austenitic alloy used for the plugs resists oxidation at high temperature and is non-magnetic.

Cast steel plugs described above are then heated, in accordance with apreferred embodiment of the invention, to a temperautre of 215 0 F. to develop scale on the outside surface which will act as a lubricant during subsequent use in seamless pipe rolling. Temperatures above about 2400. F. are avoided to preclude excessive oxidation and burning. After heating to form scale, which depending on plug size may take as much as up to 4 hours, or more, plugs are air cooled out of the furnace, preferably in quiescent air, i.e. not fan cooled, to a temperature below about 900 F. The plugs are then reheated to 1550 F. to austenitize same and held for about 4 hours to achieve temperature uniformity. After austenitizing, the plugs are air cooled out of the furnace again, preferably in quiescent air, to a temperature suitable for handling which is usually to room temperature. The plugs produced as described above have hardness after heat treatment between 390 and 45 0 B.H.N.

Comparisons of the mandrel plugs made and treated in accordance with the invention with mandrel plugs of similar but not identical compositions provided With a similar heat treatment, indicate the improved results obtained by the invention. In one comparative test proram man re rlu s in .asqw n w t he invention had TABLE I C 1.5-1.6 Mn 4'5-. 55 P 05 max S 05 max Si 1.0-1.25 Ni 6.0-7.0 Cr 2 18.0-19.0

As can be seen, the somewhat lower carbon content and the addition of aluminum and copperinthesteel used in accordance with the invention most significantly affect the life of steel mandrel plugs made therefrom. .Iclaim; Y 1. -A method of treating a steel mandrel plug for seamless pipe' rollingtoincreaseits usable life comprising heating a steel mandrel plugvconsisting essentially of 1.1 to 1.4% carbon, 6.5 to 8.5% nickel, 18.0 to .0% chromium, 1.0 to 1.5% molybdenum, 0.5 to 1.0% copper, 0.5 to 0.8% aluminum, 0.7 to 1.3% silicon, 0.4 to 0.6% manganese, up to 0.05% phosphorus and up to 0.05% sulfur to a temperautre above about 2100" F. to form scale on the surface thereof, air cooling to below about 900 F., reheating to above about 1525 F. to austenitize said steel, and then air cooling to a temperautre for handling.

2. A method in accordance with claim 1 wherein said steel mandrel plug is heated to between about 2100 and 2400 F. to form scale on the surface thereof.

3. A method in accordance with claim 1 wherein said steel mandrel plug is reheated to between about 1525 F. and about 1600 F. to austenitize same.

4. A method in accordance with claim 1 wherein said steel mandrel plug is finally cooled to room temperature for handling.

5. A method in accordance with claim 1 wherein said air cooling is performed in substantially quiescent air.

6. A steel mandrel plug for seamless pipe rolling consisting essentially of:

Percent Carbon 1.1-1.4 Nickel 6.5-8.5 Chromium 18.0-20.0 Molybdenum 1.0-1 .5 Copper 0.5-1.0 Aluminum 0.5-0.8 Silicon I 0.7-1.3 Manganese 0.4-0.6 Phosphorus Up to 0.05

Sulfur Up to 0.05

flO

. 4 and the balance iron; said steel mandrel plug further characterized by an oxide surface scale, a hardness of between 390 and 450 B.H.N.', and a stable austenitic structure.

7. A steel mandrel plug substantially as described in claim 6 but 'consistingessentially of:

. Percent Carbon 1.3-1.4

Nickel 7.0-7.5 Chromium 19.0-19.5 Molybdenum 1.0-1.5 Copper 0.650.8 Aluminum 0.5-0.7 Silicon Q. Up to 1.0 Manganese Up to 0.5 Sulfur -s Up to 0.05 and the balance iron.

References Cited UNITED STATES PATENTS 2,237,379 4/1941 Trantin '7512 6 X 2,263,020 11/1941 Trantin -128 2,270,483 1/1942 Trantin 75--l25 2,478,723 8/19'49 Trantin 75l24 2,478,724 8/1949 Trantin 75--124 2,353,688 7/1944 Burgess 148136 X 2,505,764 5/1950 Goller 148136 X 2,528,637. 11/1950 Clarke 148136 X 2,671,726 3/1954 Jennings 148-136 X 2,706,696 4/1955 Payson 148136 X 3,071,460 1/19 63 Torczyn 148-136X 3,235,417 2/196'6 Roy et al. 148136 FOREIGN PATENTS 9/ 1963 Great Britain.

OTHER REFERENCES Melals'Handbook, 1948 ed., pub. by ASM, p. 561.

Metals Handbook, 1961, 8th ed., pubiby ASM, pp. 481-485. 7 Heat Treatment and Physical Properties of the Chromium-Nickel Stainless Steels, New York: International Nickel Co. Inc., 1947, pp. 9-11.

CHARLESN. LOVELL, Primary Examiner s. c1.'X.R. 

